Dow Chemical Co. will close the former Rohm and Haas chemical plant in Philadelphia's Bridesburg section, the company told its remaining workers at the 63-acre complex this morning.

Some 25 employees at the Jetted Copolymer Facility, which makes ion-exchange materials for water filtration systems the company builds elsewhere, will lose their jobs when production ceases "by the middle of 2010," company spokesman Bob Plishka confirmed to me by phone.

The other 20 workers, at an herbicide unit that Dow's AgroSciences division bought from Rohm and Haas two years ago, had already been slated to leave when the unit closes Aug. 31.

Plishka said Rohm and Haas hasn't decided whether to sell the site, which once employed hundreds. "After production ends, our environmental (and) decommissioning process will begin."

NEW: "Fixed costs at the site are too high" to keep the copolymer unit going all by itself, Plishka said later in an email. Rohm and Haas has operated on the site since at least the 1920s. The company employs around 2,500 at its Center City headquarters and other plants in the Philadelphia area.

Dow bought Rohm and Haas in April after trying to rescind a sale agreement that looked increasingly expensive as the economy slowed and stock prices fell. Lawyers for major investors including the Haas family, Philadelphia philanthropists who inherited shares from the company's cofounder, forced Dow to go through with the deal, even though Dow warned it would have to cut expenses more than had originally intended.

EARLIER: The decision follows DuPont Co.'s closure of another city industrial landmark, Marshall Laboratory, formerly the company's Philadelphia works, earlier this summer. The company transferred workers from the specialty paint plant, which opened in the 1860s, to locations in the Wilmington area.